1. Field of the Invention
This invention relates generally to semiconductor processing, and more particularly to methods of inspecting circuit structures for structural defects using forced vibration thereof via sonic pulses.
2. Description of the Related Art
Accurate and reliable defect inspection is vital to successful modern integrated circuit fabrication. Many current integrated circuits now routinely contain millions of individual transistors, resistors and other types of circuit components. The patterning of just a few such components on a given circuit may involve scores or even hundreds of different process and wafer movement steps. Therefore, it is desirable to identify defective structural components and fabrication processes so that defective parts may be reworked or scrapped and process recipes adjusted as necessary. Because the processing of an integrated circuit proceeds in a generally linear fashion, that is, various steps are usually performed in a specific order, it is desirable to be able to identify the locations of defects as early in a semiconductor process flow as possible. In this way, defective parts may be identified so that they do not undergo needless additional processing.
The types of structural defects observed in semiconductor circuit structures are legion. One such example is catastrophic structural failure in patterned polysilicon lines. Such patterned structures are used for transistor gate electrodes, local interconnect structures, and power rails to name just a few. The failure mechanisms for such structures vary, and often include an actual rip-out or breaking away of the structure. The origins of such failures may be traced to film contamination, unintended void formation or crystalline defects. Structural weakness in the film due to any of these mechanisms may result in structural failure during processing steps that impart stresses to the wafer, such as thermal shocks associated with bath processes, chemical mechanical polishing, and plasma etching to name just a few.
Conventional techniques for identifying structural defects usually rely on some type of imaging of the structure of interest. Techniques such as optical microscopy, scanning electron microscopy and x-ray diffraction are useful in identifying certain types of structural defects. However, the latter two techniques are generally destructive of the circuit structure and thus require test wafers or sacrifice of the tested wafer, and neither of the three can pick up some types of highly localized or otherwise obscured crystalline defects. These more latent types of defects may not reveal themselves until the integrated circuit is stressed thermally or otherwise much later in a process flow.
The present invention is directed to overcoming or reducing the effects of one or more of the foregoing disadvantages.
In accordance with one aspect of the present invention, a method of detecting structural defects in a circuit structure is provided. A natural frequency of the circuit structure is determined and the circuit structure is immersed in a liquid. A first plurality of sonic pulses is sent through the liquid. The first plurality of sonic pulses have a first frequency range selected to produce a plurality of collapsing bubbles proximate the circuit structure. The collapsing bubbles produce a second plurality of sonic pulses that have a second frequency range near or including the natural frequency of the circuit structure whereby the second plurality of sonic pulses causes the circuit structure to resonate. The circuit structure is inspected for structural damage.
In accordance with another aspect of the present invention, a method of inspection is provided that includes determining a natural frequency of a polysilicon line structure and immersing the polysilicon line structure in a liquid. A first plurality of sonic pulses is sent through the liquid. The first plurality of sonic pulses have a first frequency range selected to produce a plurality of collapsing bubbles proximate the polysilicon line structure. The collapsing bubbles produce a second plurality of sonic pulses that have a second frequency range near or including the natural frequency of the polysilicon line structure whereby the second plurality of sonic pulses causes the polysilicon line structure to resonate. The polysilicon line structure is inspected for structural damage.
In accordance with another aspect of the present invention, a method of inspection is provided that includes determining a natural frequency of a trench structure in a substrate and placing the substrate in a liquid so that at least the trench structure is immersed therein. A first plurality of sonic pulses is sent through the liquid. The first plurality of sonic pulses has a first frequency range selected to produce a plurality of collapsing bubbles proximate the trench structure. The collapsing bubbles produce a second plurality of sonic pulses that have a second frequency range near or including the natural frequency of the trench structure whereby the second plurality of sonic pulses causes the trench structure to resonate. The trench structure is inspected for structural damage.